In the design and manufacture of a semiconductor device, there has been an increasing demand for a higher operating speed and a higher level of integration. Meanwhile, it has been pointed out that electro-migration (EM) easily occurs due to a current density increase caused by a high-speed operation and wiring miniaturization, whereby wiring disconnection may be caused. This results in deterioration of reliability. For this reason, Cu (copper), Ag (silver) or the like having a low resistivity has been used as a wiring material formed on a substrate of the semiconductor device. Especially, since the copper has a resistivity of about 1.8 μΩ·cm and is expected to exhibit high EM tolerance, it is regarded as a material suitable for achieving the high speed of the semiconductor device.
In general, a damascene method has been utilized to form a copper wiring on the substrate, and this method involves forming a via and a trench on an insulating film by etching, and then filling them with a Cu wiring. Further, there has been made an attempt to enhance the EM tolerance of the semiconductor device by coating a metal film called a cap metal on the Cu wiring by electroless plating by means of supplying a plating solution containing CoWB (cobalt. tungsten.boron), CoWP (cobalt.tungsten.phosphorus), or the like on the surface of the substrate having the Cu wiring (see, for example, Patent Document 1).
The cap metal is formed by supplying the electroless plating solution on the surface of the substrate having the Cu wiring. For example, the substrate may be fixed on a rotary support, and by supplying the electroless plating solution while rotating the rotary support, a uniform liquid flow is generated on the substrate surface, whereby a uniform cap metal can be formed over the entire substrate surface (see, for example, Patent Document 2).
As for the electroless plating, however, it is known that a precipitation ratio of metal is largely affected by reaction conditions such as the composition and the temperature of the plating solution, and the like. Moreover, there has occurred a problem that by-products (residues) due to the plating reaction are generated in the form of slurry and remain on the substrate surface, impeding the uniform flow of the plating solution and making it impossible to replace the deteriorated electroless plating solution with new one. As a result, the reaction conditions on the substrate become locally different, making it difficult to form a cap metal having a uniform film thickness over the entire surface of the substrate. In addition, the substrate surface on which the cap metal is to be formed becomes to have a locally hydrophilic region or a locally hydrophobic region due to a difference in the surface material or sparseness or denseness of wiring. As a result, the electroless plating solution cannot be supplied onto the entire region of the substrate in a uniform manner, resulting in a failure of forming the cap metal having a uniform film thickness over the entire surface of the substrate.    Patent Document 1: Japanese Patent Laid-open Publication No. 2006-111938    Patent Document 2: Japanese Patent Laid-open Publication No. 2001-073157